Quadrupole ion traps according to Paul and Steinwedel (DE-PS 944 900) consist of ring and end cap electrodes between which an essentially quadrupolar storage field is generated by applying RF voltages to the ring and end caps. Ions with varying mass-to-charge ratios (m/q) can be stored at the same time in this field (for the sake of simplicity, only "masses" instead of "mass-to-charge ratios" will be referred to in the following since, in ion traps, one is predominantly only concerned with singly-charged ions).
In such ion traps, the ions can be excited in resonance with the mass-specific frequency of their secular oscillation by applying an RF excitation field to the ion trap end caps as described in an article entitled "The Three-dimensional Stabilization of Charge Careers in a Quadrupole Field", E. Fischer. Inaugural Doctoral Thesis, University of Bonn, 1958. This excitation causes ions with a specific mass to absorb energy from the field and to enlarge their oscillation amplitudes. The enlarged oscillation amplitude causes the ions to leave the ion trap through perforations in one of the end caps and the ions which leave can be detected outside the trap with an ion detector as described in an article by G. Rettinghaus, Center for Applied Physics v. 22, p. 321 (1967). For measurement of the mass spectra, ions with different masses are sequentially brought into a resonance condition of this kind by changing the quadrupole RF storage field so that the ions are ejected mass by mass.
Physically intrinsic resonance conditions of the storage field are preferably used to increase ion ejection. With a pure quadrupole field, resonance conditions of this kind are found at the edge of the stability zone in the a,q diagram as described in EP-A1 0 350 159. In addition, certain nonlinear resonance conditions in particular, those which occur in the case of a superposition of multipole fields, can also be used for ion ejection. For example, the use of a nonlinear octopole resonance, .beta..sub.z +.beta..sub.r =1, applied after the initial push of the secular oscillation with a fixed-phase frequency of precisely one third of the storage frequency is well-known for ion ejection as described in EP-A1 0 383 961.
FIG. 1 shows some known storage field resonance conditions both for a pure quadrupole field and for superposed hexapole and octopole fields plotted on an a,q stability diagram. The storage field resonances, .beta..sub.z =1 (for pure quadrupole), .beta..sub.z =2/3 (for hexapole superposition), .beta..sub.z +.beta..sub.r =1 and .beta..sub.z =1/2 (both for octopole superposition), have been plotted. The following applies in the customary manner: EQU a=-8zU/(mr.sub.0.sup.2 .omega..sup.2), q=4zV/(mr.sub.0.sup.2 .omega..sup.2)
where:
z=Coordinate of the rotationally symmetric axis of the ion trap, PA1 U=Direct voltage with which the RF storage field is superposed, PA1 m=Mass of ions, PA1 r.sub.0 =Inside radius of the ring electrode, PA1 .omega.=Angular frequency of the storage RF, and PA1 V=Amplitude (voltage) of the storage RF
The advantages of these superposed multipole fields are discussed in detail in the International Journal of Mass Spectroscopy Ion Processes, J. Franzen, v. 106, pp. 63-78 (1991) which article is hereby incorporated by reference. Resonance conditions at the center of the stability zone can also be produced by adding RF alternating fields with frequencies f&lt;F/2 where F is the frequency of the storage field.
In contrast to the exponential rise in ion oscillation amplitude caused by ion absorption of energy from various storage field resonances, the absorption of energy by the ions from the excitation field causes their secular oscillation amplitudes to increase only linearly. Therefore, ion ejection caused by storage field resonances is very much sharper and can be concluded in fewer oscillation cycles.
The secular oscillation frequency of the ions varies widely after their production or introduction. Consequently, in order to provide a well-resolved mass spectrum, it is necessary to first collect the oscillating ions confined in the ion trap near the center of the ion trap to enable the ions of successive masses to leave the ion trap in ejection cycles clearly separated from each other in terms of time. For this, the ion trap is preferably filled with a special damping gas having an optimal density enabling the ions to release energy by colliding with the remaining gas in the trap. When such a gas is introduced, the trapped ions "thermalize" after a few collisions and collect at the center of the quadrupole field due to the focusing effect of the quadrupole field, reducing their oscillation amplitudes at the same time. They form a small cloud, the diameter of which is only approximately 1/20 to 1/10 of the dimensions of the trap according to tests carried out with laser beams as described in Physical Review A, I. Siemers, R. Blatt, T. Sauter and W. Neuhauser, v. 38, p. 5121 (1988) and Journal of the Optical Society of America B, M. Schubert, I. Siemers and R. Blatt. v. 6, p. 2159 (1989). Thermalization takes place particularly quickly with medium-weight damping gas molecules such as air.
If the ions of a selected mass are now coherently pushed out of the cloud under the foregoing resonance conditions, they absorb further energy practically synchronously. If the diameter of the ion cloud of a selected ion mass does not greatly enlarge, but the oscillation amplitude markedly increases, all ions of the selected mass then leave the ion trap in just a few oscillation cycles. By experiment, the ions of a mass can be practically completely ejected in approximately 5 to 7 oscillations, utilizing storage field nonlinear resonances. This provides a very good mass resolution, even with very fast scanning methods.
Measurement of the ions leaving the ion trap is customarily carried out with a secondary-emission multiplier, providing practically delay-free amplification of the ion signal by a factor of 10.sup.5 to 10.sup.7. The outgoing electron emission current of the secondary-emission multiplier is usually fed, via an impedance converter (electrometer amplifier), to a clocked digitizing stage (analog/digital converter or ADC). The size of the bandwidth of the impedance converter is selected in such a way that the intensity progression via the ion stream profile of a mass is spread as little as possible. However, the ion signals, which occur like pulses, are to be integrally smoothed to the point that a fairly monotonic progression of the ion stream profile of a mass is produced. In doing so, the clock pulse of the converter is matched to the bandwidth of the impedance converter in such a way that the digital numbers generated reflect the signal progression without considerable losses. The string of digits forms the "raw spectrum" which can be processed further in a data system in known manner.
This method also provides satisfactory results for a slow mass scan, in which the ion profile of a mass consists of a great number of individual ion packages. For fast scans, however, which consist of only 5 to 7 ion packages, as described above, spectra recorded in this manner have two serious drawbacks: Firstly, signals for individual ion masses have much greater fluctuations than are to be anticipated according to statistical expectations based on ion numbers.
Secondly, the signals have relatively marked background noise due to ions taken out of their orderly ejection rhythm by collisions with molecules of the damping gas. This background noise obstructs the identification of very small ion signals. The background noise is also present with the slow scan but is less conspicuous due to the overall poorer level of detection.
It is the task of this invention to reduce both signal fluctuations and background noise in the spectrum measured. For this, it is necessary to understand the reasons for the signal fluctuations and noise background.
More particularly, the ions are ejected from the ion trap at regular intervals. Each time the oscillating ion cloud reaches the perforated end cap, enlarging its oscillation amplitude by absorbing energy from the storage field, an ion package from the front of the cloud reaches the holes and escapes through them. By means of an oscillograph, it is possible to establish that the ion packages (or ion pulses) are very short in terms of time, only lasting approximately 30 to 100 nanoseconds. Since the secular frequency cycle is approximately 3 microseconds, the current pulses of the ion packages occupy only 1/100 to 1/30 of the total scan time.
Despite the very brief time between resonance start and ejection of the ion packages, some unavoidable disruptive collisions between the oscillating ions and the damping gas take place in the ion trap. The disturbed ions assume disorderly forms of trajectories and are able to evade orderly ejection. They are, however, able to leave through the holes in the further course of the scan and are detected by the ion detector. Since these ions are no longer related to the orderly-ejected ions, they form a disturbing noise background. Since they no longer move coherently to the ions of a mass, they are uniformly distributed across the scan time, in particular thus also appearing between the individual orderly ion pulses.
In addition, it is no longer possible to produce a good compromise for measurement of the ion stream profile of an ion mass only consisting of 5 to 7 ion packages by the setting of the bandwidth of an amplifier. The demand for integral smoothing of signals requires a slow amplifier. A fast mass sequence, however, necessitates a very fast amplifier. A fast amplifier inevitably results in an ion signal appearing to be broken up which, in connection with the clock-pulse rate of the digitizer, leads to erratic sequences of numbers, from which it is scarcely possible to identify the signal profile of the ion masses.